Supercapacitors generally comprise two porous electrodes impregnated with an electrolyte (an ionic salt in generally organic solution, a quaternary ammonium salt, such as tetraethylammonium tetrafluoroborate in acetonitrile or propylene carbonate, for example). These electrodes are generally separated by an insulating and porous membrane allowing the circulation of the ions of the electrolyte.
The first supercapacitors, known as “EDLCs” (Electrochemical Double Layer Capacitators), are based on a principle equivalent to that of conventional capacitors with polarizable electrodes and an electrolyte acting as dielectric. Their capacity originates from the arrangement of a double layer of ions and of electrons at the electrolyte/electrode interface. Today, supercapacitors combine, for energy storage, a capacitive component resulting from the electrostatic arrangement of the ions close to the electrodes and a pseudocapacitive component due to oxidation/reduction reactions in the capacitor.
The electrostatic component of the energy storage is produced by a nonhomogeneous distribution of the ions of the electrolyte in the vicinity of the surface of each electrode, under the effect of the difference in potential applied between the two electrodes. The electrostatic component of the energy storage confers a potentially high specific power and a very good behavior throughout the charging and discharging cycles.
Materials having a very high ratio of specific surface to volume, having a porosity suited to ion storage at this scale, have been developed in order to increase the capacity of supercapacitors. The methods for manufacturing these materials have been directed towards the use of fullerenes, carbon nanotubes, activated carbon, carbon nanofibers or CNFs and graphene, which are advantageously light, inexpensive and less toxic than the materials commonly used for producing batteries.
Supercapacitors might replace conventional capacitors for applications having a high energy demand, especially having extreme temperatures, vibrations, high accelerations or a high salinity. In these environments, batteries may not operate without their lifetime being greatly restricted (these conditions apply to radars, to motor sports, to electrical avionics and to military applications, for example).
Supercapacitors are mainly applied to systems which require energy peaks over short times (i.e. with high power), of the order of a minute, for phases of acceleration of vehicles in ground transportation (motor vehicles, tramways, buses, “stop and start” devices, in which energy is recovered during the deceleration).
Supercapacitors might also be useful for the management of electricity in onboard systems, for rendering electrical installations secure, for rendering the energy supply of sensitive systems (radio sets, monitoring systems, military field, data centre) secure, in networks of self-contained sensors for applications in monitoring industrial, complex or sensitive sites (hospitals, avionics, offshore platform, oil prospecting, underwater applications) and finally in renewable energies (wind power, recovery of atmospheric electrical energy).
In order to enable an industrial application, the energy density and the power of supercapacitors have to be optimized. Furthermore, the internal resistance of a supercapacitor is currently too high and poorly controlled. The usual supercapacitors are composed of activated carbons with nonhomogeneous and nonoptimized distributions of the size of the pores and use a polymeric binder to ensure the mechanical strength of their structure. This binder increases the internal electrical resistance of the capacitor and disadvantageously increases its weight. Furthermore, this binder breaks down over time and pollutes the electrolyte by degrading the performance of the supercapacitor. The unsuitable porosity also produces a resistance to ion transfer within the active material.
The publication by Bondavalli, P., Delfaure, C., Legagneux, P., Pribat, D., 2013, “Supercapacitor electrode based on mixtures of graphite and carbon nanotubes deposited using a dynamic air-brush deposition technique.”, Journal of The Electrochemical Society, 160(4), A601-A606, discloses a process for the deposition of graphene nano/microparticles and of carbon nanotubes by hydrodynamic spraying of a suspension over a support. This process makes it possible to produce supercapacitors achieving high energy and power densities, without use of a polymeric binder, but requires the use of toxic and polluting solvents, such as N-methyl-2-pyrrolidone (NMP) in order to enable the suspension of the nano/microparticles. The solvents used are however suitable for suspending this type of micro/nanoparticles.
The publication by Bondavalli et al. also discloses the production of collectors on surfaces of the order of a cm2. The spraying operation is carried out using a nozzle that sprays a suspension composed of several types of mixed micro/nanoparticles. In order to cover this surface, the substrate is fixed and the spraying jet of a nozzle may be displaced in a plane so as to cover a surface by adjusting the trajectory of the jet so as to have as uniform a deposition as possible of micro/nanoparticles.
The evaporation of toxic solvents such as NMP may be confined to the laboratory during production of small surface areas, for example by carrying out the deposition under a hood. Treatment of the toxic emissions poses a technical challenge if the rate of production of the collectors, electrodes or capacitors is scaled up to the industrial level.
Producing a supercapacitor may require the deposition of several layers of different compositions of micro/nanoparticles. Using the technique disclosed in Bondavalli et al., the nozzle must be changed or cleaned during each of the steps of deposition of different micro/nanoparticles or of layers having different compositions of micro/nanoparticles.
The nozzles are also subject to clogging. The concentrations of micro/nanoparticles may be high during a spraying operation carried out under the conditions described by Bondavalli et al.: clogging poses a maintenance challenge during industrial use of this method for the production of collectors, electrodes or supercapacitors.